77 Carrera 3 Bucking on decelleration from 3000rpm down

[dave944cab]

Hi all,

I have a problem with my 77 3.0 l carrera. The car drives great when giving it trottle but bucks forward and back on deceleration from 3000rpm down. It also bucks a little under very slight trottle up to 300rpm.

I have tried the basics and they have improved the driving of the car but the bucking remains all be it a little less i think. Below is what I have tried

New points and set the gap
New ht Leads & spark plugs ( Gap set )
New Rotor arm and Dist Cap
New Fuel filter
Added several different fuel additives with no difference.
Checked control pressure and fuel Pressure when cold and hot -- looks normal
Removed each injector and checked spray pattern all look good -cleaned with carb cleaner.
Set the timing with timing light.

If anyone has any ideas i will try anything, I have spent lots of time and money tring to get it right and i will continue to try until its solved. Any help its greatly appreciated.

Dave
77 carrera 3
78 928
79 204bhp Sc

[dave944cab]

This "Bucking" is on 3 seperate threads that I have searched for. I have emailed 2 of the guys that had it and they still have it.

Lots of pepole are saying Bosch called it " Bonanza" and its a trate of the 911. I have had 15 911's from 1976 to 1979 and I have had 2 that did it.

One thing I am sure of is that there is no way Porsche had 911's Bucking from the showroom.

Has Anyone had this and found a solution.

Dont know what to try?

I know there is huge knowledge here, there has to be someone who knows

Dave

[spuggy]

Originally Posted by dave944cab

It also bucks a little under very slight trottle up to 300rpm.

Does it "surge" climbing hills on a constant throttle opening below 3,000?

Sounds like the idle mixture might be too lean, the C02 reading might need to be fattened up.

Or look at Souk's thread on testing/adjusting CIS mixture at idle by moving the air plate.

CIS Idle Speed and Mixture Setting Without an Analyzer

HTH.

[dave944cab]

It doesn't surge at all. when i drive below 3000rpm with only the slightest amount of throttle I get this bucking. If I start with plenty of throttle then there is no problem.

Same thing when i decelerate down with no throttle, when the revs drop below 3000 rpm it can start bucking.

[john walker's workshop]

76/77 2.7s often smooth out when the side vacuum hose is removed from the warmup regulator.

[dave944cab]

Thanks

Will try it tomorrow. Has to be worth a go at this stage.

[kidrock]

I don't know squat about CIS, but I would recommend following John Walker's advice.

FWIW, I have also heard that a "lean" mixture will cause "bucking" in our cars. If removing the vacuum hose to the WUR doesn't cure your problem, I think richening your mixture may be your next step.

[HenrikL]

Do you have a rubber-centered clutch disc? This behavior sounds exactly like a 944 I used to own, where the clutch rubber center was bad.

[evill]

Originally Posted by john walker's workshop

76/77 2.7s often smooth out when the side vacuum hose is removed from the warmup regulator.

John, I imagine you would re-set the idle CO after removing and plugging the vacuum hose, correct?

Is it the vacuum actuated transition from lean idle / cruise to the richer full load mixture that causes the stumble or surge?

Regards,
Ed